Ion implantation is a standard technique for introducing conductivity—altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
In one prior art approach, a high current, broad beam ion implanter employs a high current density ion source, an analyzing magnet to direct a desired species through a resolving slit and an angle corrector magnet to deflect the resulting beam, while rendering the beam parallel and uniform along its width dimension. A ribbon-shaped ion beam is delivered to a target, and the target is moved perpendicular to the long dimension of the ribbon beam to distribute the ion beam over the target.
The delivery to a semiconductor wafer of a parallel ion beam at a known incidence angle is an important requirement in many ion implantation applications. A parallel ion beam is one which has parallel ion trajectories over the surface of the semiconductor wafer. In cases were the ion beam is scanned, the scanned beam is required to maintain parallelism over the wafer surface. The parallel ion beam prevents channeling of incident ions in the crystal structure of the semiconductor wafer or permits uniform channeling in cases where channeling is desired. In addition, a parallel ion beam at a known incidence angle is required in tilted implant applications to ensure uniform results. These requirements have made it necessary to measure beam parallelism and direction and to adjust these parameters if necessary. Techniques for adjusting beam parallelism in ion implanters are disclosed in U.S. Pat. No. 6,437,350, issued Aug. 20, 2002 to Olson, et al.
One known approach to measuring ion beam angle is disclosed in U.S. Pat. No. 6,791,094, issued Sep. 14, 2004 to Olson et al. An object is placed in the ion beam, and the size and relative position of the shadow cast by the object is measured. An ion beam incidence angle and beam divergence monitor is disclosed by Larsen et al. in U.S. Patent Publication No. 2002/0121889 A1, published Sep. 5, 2002. The measurement device uses an aperture and a variable resistor to measure implant angle. Both of the disclosed techniques have a limitation in that they are capable of providing angle information in only one dimension. Moving the apparatus across the beam permits measurement only in the direction of motion. To make measurements in another direction, an additional or more complex mechanism is necessary to drive the object or slit in the desired direction.
Additional techniques for measuring ion beam angle are disclosed in U.S. Pat. No. 5,039,861, issued Aug. 13, 1991 to Swenson; U.S. Pat. No. 5,180,918, issued Jan. 19, 1993 to Isobe; and U.S. Pat. No. 5,898,179, issued Apr. 27, 1999 to Smick et al. All of the known prior art ion beam angle measuring techniques have had one or more disadvantages, including limited angle measuring capabilities, lack of accuracy and high cost.
Accordingly, there is a need for new and improved methods and apparatus for measuring ion beam incidence angles.